1. Field of the Invention
The present invention relates to a method of producing a porous ceramic filter formed of a cordierite composition, and more particularly to a method of producing such ceramic filter suitable for removing soot or other particulate matter contained in exhaust gases emitted by a diesel engine.
2. Discussion of the Prior Art
Recently, various types of porous honeycomb filters using a cordierite honeycomb structure with porous partition walls have been proposed as a porous ceramic filter capable of functioning to filter fluids such as gases. For example, porous honeycomb filters are used as a so-called "diesel particulate filter" for removing particulate matter from exhaust gases emitted by a diesel engine. These diesel particulate filters are roughly classified into a high-trapping-efficiency type, and a low-trapping-efficiency type. These two types are selectively employed depending upon the specific requirement.
As a porous ceramic honeycomb filter having improved filtering capability, laid-open Publication No. 61-129015 of unexamined Japanese Patent Application proposes an exhaust emission purifying filter in which the size of the pores formed adjacent to the surfaces of the partition walls of the honeycomb structure is specifically controlled. Described more particularly, those pores consist of relatively small pores whose diameters fall within a range of 5-40.mu.m, and relatively large pores whose diameters fall within a range of 40-100.mu.m. This exhaust emission purifying filter is prepared from a ceramic composition with which a suitable foaming or blowing agent is mixed. A desired green honeycomb structure formed of this ceramic composition is fired at an elevated temperature, whereby pores are formed in the partition walls of the fired honeycomb structure, due to heating of the ceramic composition in the presence of the foaming agent mixed therein.
Laid-open Publication No. 61-54750 of examined Japanese Patent Application discloses porous honeycomb filters in a wide range of trapping capacity from a high--trapping-efficiency type to a low-trapping-efficiency type. The porous honeycomb filters disclosed therein have controlled open porosities (ratio of a volume of open pores to that of non-open pores) and controlled average sizes of the pores. Further, laid-open Publication No. 58-70814 of unexamined Japanese Patent Application teaches that the pressure loss of a porous ceramic honeycomb filter can be lowered by forming the partition walls of the honeycomb structure with large pores having 100 .mu.m or larger sizes, for example.
Generally, the following three characteristics are important in determining the overall filtering function or capability of a porous ceramic honeycomb filter. These characteristics arc: a) trapping efficiency (ratio of the particulate matter removed from a subject fluid, to those not removed); b) pressure loss (amount of pressure drop of the subject fluid flowing through the filter); and c) nominal operation time (time duration from the commencement of use of the filter to the time at which the pressure loss increases to an upper limit). In this respect, it is significant to note that the trapping efficiency is proportional to the pressure loss. Namely, an increase in the trapping efficiency results in an undesirable increase in the pressure loss, and a consequent decrease in the operation time. If the filter is adapted for a comparatively reduced amount of pressure loss, the operation time can be prolonged, but the trapping efficiency is unfavorably lowered.
The most important characteristic of the porous ceramic honeycomb filter is the trapping time, i.e., the time duration for which the filter can operate with the pressure loss held below the permissible upper limit. For the reason indicated above, however, it has been considered difficult to increase the trapping time while maintaining a sufficiently high trapping efficiency. In this respect, it is noted that an increase in the nominal operation time of a porous ceramic honeycomb filter means a decrease in the required volume of the filter for a specific application, and the decrease in the required volume contributes to an improvement in the thermal shock or stress resistance of the filter. Therefore, it is desirable to increase the operation time (life expectancy) of the filter, particularly where the contaminated or clogged filter can be reclaimed by burning out the contaminants or particulate matters, as in the case of the diesel particulate filter used for a diesel engine.